1. Field of Invention
This invention relates to air-breathing, hydrogen fueled jet engines and specifically to a new hydrogen expanding mechanism to improve the efficiency and thrust of such engines.
2. Discussion of Prior Art
The air compressor of a conventional turbojet engine is powered by a turbine which expands combustion products. The turbine limits the turbojet in two ways. Firstly, as the combustion products expand through the turbine, the propulsive energy of the combustion products is greatly reduced. Secondly, turbine materials are temperature limited, which limits the maximum combustion temperature. When an aircraft accelerates to a sufficiently high supersonic speed, the temperature of air entering the combustion chamber will approach the temperature limit, and so little or no fuel can be burned without overheating the turbine.
A hydrogen fueled jet engine can obviate the turbine which expands combustion products. Instead of expanding combustion products, high pressure hydrogen is expanded in a turbine to produce the power needed for the air compressor. After the hydrogen expands to produce power, it is mixed with the compressed air in a combustion chamber where it is burned as fuel. Since there is no turbine to expand combustion products, all the energy of combustion contributes to propulsive thrust. Since there is no turbine to overheat in the combustion products, combustion temperature can be greatly increased.
U.S. Pat. No. 2,620,625 to Phaneuf, Dec. 9, 1952 describes a radial inflow reaction turbine driven by hydrogen. This turbine directly drives a centrifugal air compressor and a centrifugal hydrogen compressor to impel the hydrogen to a combustion chamber. U.S. Pat. No. 3,000,176 to Kuhrt, Sep. 19, 1961 describes a hero's wheel turbine (named for Hero, the ancient inventor). This turbine directly drives an axial flow compressor. U.S. Pat. No. 3,705,496 to Wolf et al, Dec. 12, 1972 and U.S. Pat. No. 3,747,339 to Wolf et al, Jul. 24, 1973 show an axial flow turbine directly driving an axial flow compressor. A heat exchanger in front of the compressor preheats the hydrogen while cooling the incoming air. Although cooling incoming air may increase compressor efficiency somewhat, it seems counterproductive to cool the air prior to heating it up again by the addition of fuel. U.S. Pat. No. 5,012,640 to Mirville, May 7, 1991 shows the hydrogen driven turbine mounted integrally with the rotor wheel of the axial air compressor stages. This engine would likely have serious problems with hydrogen leakage at the seals.
The abovementioned patents are all very inefficient because the sonic speed of sound in heated hydrogen is about five times that of the incoming air. A hydrogen turbine therefore needs to spin much faster than an air compressor. The following patents use a gearbox to allow for this.
U.S. Pat. No. 2,956,402 to Rae, Oct. 18, 1960 shows a three stage turbine with reheat between stages. Although theoretically a little more efficient than a single stage turbine (without reheat), the complexity makes it impractical. U.S. Pat. No. 3,237,401 to Peters et al, Mar. 1, 1966 shows a hydrogen turbine, gearbox, air compressor, and a hydrogen to hydrogen heat exchanger. Warm hydrogen exiting the turbine is cooled while passing through the heat exchanger, while the cold liquid hydrogen from the fuel tank is gasified and preheated. Although the heat exchanger is stated to be lightweight, it seems counterproductive to cool the fuel just before burning it for heat value. U.S. Pat. No. 3,241,311 to Kuhrt, Mar. 22, 1966 shows a turbine driving a compressor through a gearbox, and two axially spaced fuel manifolds (injectors). A proportioning valve between the two fuel manifolds helps control overheating. The Model 304 Engine (described in NASA SP-4404, chapter 8–9) is quite similar to U.S. Pat. No. 3,241,311. To the inventors knowledge, the Model 304 is the only hydrogen expanding jet engine to actually be built and tested. An 18 stage hydrogen expanding turbine powered a four stage axial flow air compressor through a reduction gear. It had two fuel injection manifolds with a heat exchanger between them. The amount of hydrogen injected and burned by the first manifold was controlled to limit the temperature of the combustion gasses which entered the heat exchanger. The remaining hydrogen was injected at the second manifold beyond the heat exchanger. The hydrogen expanding turbine was very highly developed, yet the claimed efficiency was about 50%, which seems rather low. The reduction gear may have been partly to blame for this. Since the Model 304 turbine was highly developed, the other hydrogen turbines referenced above would undoubtedly be much less than 50% efficient.